Friday, May 23, 2008

Cleaning up our test code – Part 2

Assuming that we've created our test object in a few moves (please read the previous post about it), now the focus switches to the way we use test code to assert correctness of production code. JUnit assertion family relies heavily on equals-based assertions. Unfortunately, the equals() method is far from being used consistently, so equals-based testing has some dangers we need to be aware of.

The equals() contract

Talking about the equals() method, there is a general behavioral contract, which is the one defined in the Java Specification, and it is used heavily in the Collections classes to retrieve objects from different container classes. As every good java developer knows, overriding equals() needs us to adhere to the implicit behavioral contract, and also that we override hashCode() to ensure consistent behavior with different container types. So, to effectively test or domain objects, we need to override both methods. So far so good.

There are also a few convenient methods to do this: Eclipse allows you to generate equals() and hashCode() from a given set of attributes. The resulting code quite good, but it's like a grey area in your source code, in terms of readability. Jakarta commons implementation is less "automatic" but provides the developer with better control over the final result.

Enter a new player

If you're using Hibernate to persist your domain objects, you'll probably know that this requires some attention on the way equals() and hashCode() are defined in your application. This is primarily tied to the hibernate persistence lifecycle (which generally populates id fields upon writing operations) and to lazy-loading (some fields are loaded only if they're explicitly accessed inside a Hibernate session). The Hibernate recommendation is to define equals() and hashCode() methods according to equality of the so-called business-key, which can be roughly be mapped to "a unique subset of required non-lazy fields, excluding id". Id-based equality should be managed only by Hibernate, while business operations should rely on an equals() method based on the business key. To purists, this sounds like an undesirable implicit dependency on the Hibernate framework (your POJOs are still POJOs, but not exactly the same POJOs you would have had without Hibernate).

Equality as a business-dependent concept

So far, we have 2 separate equality implementations: id-based equality (that should be used only bi Hibernate, behind the scenes) and business-key equality that will be used in our business code and will be implicitly used if we uses containers from the Collections framework. What should we use in testing? Unfortunately, there is no one-size-fits-all answer, but the choice depends heavily on what we are testing and what is precisely the desired behavior of your application. If we are adding information on some non-mandatory field, then simple equality won't check it. If we're changing the value of a mandatory field, and want to check that this doesn't trigger creation of a new Id, you need to explicitly check that field.

Often, applications with a nontrivial domain model can't rely only on a single notion of equality (are two BankAccount instances equal if they have a different balance?), this is more or less clear, during analysis, but the presence of an assertEquals() method in JUnit makes blindly using equals()so tempting…

Smarter predicates

Once we've realized that equality is too generic to be applied blindly, the following step is to try to apply the right context-dependent equality in the appropriate context. The obvious solution to do this is to decompose equality to an attribute-level check: so instead of having

assertEquals(a, b);

we end up with something like

assertEquals(a.getName(), b.getName());assertEquals(a.getSurname(), b.getSurname());// … you got the point

Which is longer, less maintainable, with a higher coupling, and … ugly. Most of the times, anyway a business relevant notion of equality doesn't show up only in tests. I would argue that 99% of the times the same equality is hidden somewhere in your code in form of some business rule. Why not having the same rule emerge and be used in the test layer as well?

A good way to do this is to rely on the Specification pattern, developed by Eric Evans and Martin Fowler which basically delegates to a dedicated object the assertion of the applicability of a given business rule on a domain object. Put in another way, Specifications are a way to express predicates or to verify expectations on domain objects, in a way that could look like:

After thoroughly testing the Specification (a thing that we should have done anyway, since it is a business implementation), we could be able to reuse the same logic as an assertion mechanism in our test layer, making our code shorter and cleaner. Not all business oriented assertions will be that useful in the test layer, but some normally do. As I said in the previous posts, one of the main goals was to be able to write a lot of tests, and to write them in few minutes. Being able to rely on a higher level of abstraction definitely helps